In order for cancer cells to proliferate and grow, new blood vessels are required to supply oxygen and nutrients. Various factors are involved in angiogenesis; however, among them, a vascular endothelial growth factor (VEGF) serves as the most important regulator (see Ferrara and Davis-Smyth (1997) Endocrine Rev. 18: 4-25; Ferrara (1999) J. Mol. Med. 77:527-543).
VEGF is a dimer configuring sub-units and having a molecular weight of about 46 KDa, the sub-unit having a molecular weight of about 23 KDa and regulates vasculogenesis at an embryogenesis as well as angiogenesis in an adult organism. Five kinds of VEGFs (VEGF-A, VEGF-B, VEGF-C, VEGF-D and PLGF) have been found so far in mammals. VEGFs is bound to co-receptors such as three receptor tyrosine kinases (RTKs) known as VEGF receptors (VEGFRs) -1, -2 and -3, heparin sulphate proteoglycans (HSPGs), and neuropilin (NRPB) in an overlapped scheme. The VEGF receptor causes cell migration, survival, and proliferation, like many growth factor receptors, and has functions of delivering a signal capable of forming a three-dimensional blood vessel or controlling a blood vessel permeability, wherein the functions are not found in the other RTKs.
It has been found through a target gene inactivation research in mice that VEGF is a factor required for an early stage of angiogenesis, wherein a VEGF molecule is upregulated in tumor cells and a receptor thereof is upregulated in tumor infiltrated vascular endothelial cells; however, in normal cells that are not involved in angiogenesis, expression of VEGF and the receptor thereof are maintained at low level (Brown et al., Cancer Res. 53: 4727-4735 (1993); Mattern et al., Brit. J. Cancer. 73: 931-934 (1996)). Therefore, VEGF which promotes formation of new blood vessels has drawn attention as a target for treating cancer.
Therefore, recently, a novel anti-cancer treatment for blocking production of new blood vessels has been developed, and an anti-VEGF receptor antibody, a soluble receptor structure, an antisense, an RNA aptamer that binds to VEGF, low molecular VEGF receptor tyrosine kinase (RTK) inhibitor, and the like, has been suggested in order to inhibit VEGF signaling (Siemeister et al., Cancer Metastasis Rev. 17: 241-248 (1998)). Subsequently, it has been found in nude mice that an anti-VEGF neutralizing antibody inhibits growth of various human tumor cell lines (Warren et al., J. Clin. Invest. 95: 1789-1797 (1995); Borgstrom et al., Cancer Res. 56: 4032-4039 (1996); and Melnyk et al., Cancer Res. 56: 921-924 (1996)).
Among patent documents related to the VEGF inhibitor, U.S. Pat. No. 6,011,003 discloses an altered, soluble form of FMS-like tyrosine kinase receptor (FLT) polypeptide including immunoglobulin domains exerting an inhibitory effect on VEGF, and WO 98/13071 discloses gene therapy for inhibiting primary tumor growth and metastasis by gene transfer of a nucleotide sequence encoding soluble receptor protein which binds to VEGF. In addition, WO 97/13787 discloses a low-molecular VEGF inhibitor usable in treatment of diseases accompanied by neovascularization, and WO 00/75319 discloses modified polypeptides including sequences of modified Flt 1 and Flt 4 which are VEGF receptor.
An angiogenesis inhibitor according to the related art inhibits an angiogenesis, which is required for cancer growth; however, since the angiogenesis inhibitor does not have a targeting function against tumor cells, a cancer cell-specific anti-cancer effect is not capable of being exerted, and side effects may occur on normal blood vessels. Subsequently, in the case of Bevacizumab (Avastin™) commercially available as a humanized antibody to VEGF, it was announced from phase III clinical trials by Genentech, Inc., that intestinal bleeding, hemoptysis, hemorrhage, epistaxis, coughing up blood as side effects were observed, and headaches, high blood pressure, nasal swelling, albuminuria, dry skin, excessive tears, back pain, skin edema, and the like, were also observed. It is considered that the above-described side effects are shown since the angiogenesis inhibitor according to the related art does not have a targeting function against tumor cells.
Therefore, it is necessary to develop an anti-cancer agent capable of effectively treating cancer by effectively delivering an angiogenesis inhibitor to cancer through selective targeting of cancer cells to reduce side effects and to effectively inhibit angiogenesis required for cancer growth, and by directly inhibiting the cancer cells.